Generally, power amplifiers as are used in the communication art exhibit non-linear output characteristics, particularly as the transmit power level is increased in response to the input signal. Non-linear behavior results in distortion in the output signal that is undesirable, and may cause the communications system to fail to meet the required performance metrics as set by standards organizations or the governing regulatory agencies. However, as is well known, any power transistor driving a load will act in a non-linear fashion when operating in the transistor saturation region, that is, the linear relationship of the voltage input-voltage output characteristic is only maintained in the linear operation region of the transistor. As the input signal power is increased to and beyond the saturation point of the driving transistors in the power amplifier, the response of the transistor(s) does not change linearly and the power output characteristic therefore inherently becomes non-linear.
An important application for non-linear elements, for example, these power amplifiers, in the current art are in the area of wideband cellular communications. A typical application for a power amplifier is a forward or transmitter function for a cellular telephone base station (of course, the base station also receives as well, however the receiver function is not addressed here). Communication is accomplished using several well known standard protocols for voice and/or data transmission, for example CDMA, WCDMA, TDMA, GSM, EDGE, 3G and the like. These applications require and will continue to require increasing amounts of data to be transmitted over the cellular system. Further the communications units may be devices other than cell phones, including wireless portable email terminals, computers both fixed and portable such as laptops and palm computers, fixed location, handheld, and vehicle mounted telephone equipment, personal internet browsing devices, even video equipment and other communications or data receiver or transmitter applications. In these applications and other applications, RF transmit power is used and applied to a power amplifier with significant power consumption, so that distortion may occur when the input signal and the power applied take the power amplifier into operating regions where the output signal is not a linear function of the input signal.
FIG. 1 depicts a simplified schematic of the RF transmit portion (sometimes called the “analog” portion) for such a prior art base station application. (Only a single RF transmit function and one antenna is shown, however, many may be used). In FIG. 1, digital baseband transmit data VFF is input to a digital to analog converter (DAC) 3, which outputs analog data for transmission to a frequency converter 5 that is clocked by a local oscillator 15. The frequency converted analog signal is next presented to a power amplifier 11, and the resulting analog amplified output signal Vout is presented to an antenna 14 for transmission. A feedback path then returns an observed version of the analog transmitted signal Vout, labeled VFB, for observation through a coupler 13. This is accomplished, for example, by coupling the antenna signal to a second frequency converter 17 which is likewise clocked by local oscillator 15, the converted analog observation signal is then processed by analog-to-digital converter 19 and digital signals VFB are made available for observation, and for use in compensation of the transmit signal using feedback techniques. (Note that the voltage VFB is not the voltage received from other transmitters at the antenna, these are sent along a different path to an RF receiver, which is not addressed here).
During operation of such an RF transmit function, it is known that as for any power amplifier where a power transistor drives a load based on an input signal, if the output signal to be generated is such that the transistor enters its saturation region, the output becomes non-linear; that is the output no longer varies linearly with the input signal. This effect is described as non-linear distortion in the output signal. A simple and somewhat effective approach to limiting non-linear power amplifier distortion known in the prior art is to “back of” the output power, so that the amplifier operates only within the linear part of its voltage response characteristic. However, to be effective this output power backoff must account for the peak-to-average ratio (PAR) of the input signal. In order to account for the PAR, and still output the correct signal, the required transmit power backoff to maintain linear performance of the driving transistors therefore becomes very significant. This results in very low amplifier efficiencies. A metric sometimes used in the art for describing the peak amplitude verses average amplitude of a signal is the term “crest factor”. As an example, a CDMA signal of the present cell phone systems may exhibit a PAR of up to 13 dB. Power backoff is a source of inefficiency in the operation of the power amplifier, because the amplifier is being intentionally operated at lower than possible output power levels for a given supply power. This amplifier inefficiency results in additional power consumption during operation of the system and therefore increases the manufacturing costs of the system, because to achieve a certain output power requires that a much more powerful amplifier be used, while if the backoff were not required, a lower gain amplifier can be used, and lower costs for the system would accrue. When “back of” is used to prevent distortion in the output, this inefficiency results in increased operating or ownership costs for the system, as the operating power expended for the actual transmit power is increased.
One prior art approach to reducing the amount of “backoff” required is to modify the input signal prior to applying it to the power amplifier by applying “crest factor reduction” or CFR. When CFR or peak amplitude reduction is applied to a signal, large amplitude peaks are identified in the input signal, a compensating signal with an opposing crest is created and then this inverted signal is combined with the input signal so as to remove only the largest peaks in the input signal, and so reduce the crest factor, i.e. reduce the PAR, prior to presenting the signal to the power amplifier for transmission. This approach advantageously reduces the amount of “backoff” required to maintain the power amplifier in its linear range, and efficiency for the system is thus increased. Processors specifically designed to provide the CFR function are commercially available, for example Texas Instruments, Inc. supplies integrated circuits designated as part no. GC1115 particularly directed to implementing CFR in communications systems. The GC1115 is a crest factor reduction preprocessor that receives digital upconverted input signals as I (in phase) and Q (quadrature phase) form digital signals, for example, and outputs a modified I,Q, signal with the peaks removed for transmission with reduced PAR. The GC1115 integrated circuit is operated by a programmable DSP executing software. Other manufacturers offer similar integrated circuit solutions for implementing CFR in commercial transmit systems as is known in the art.
In addition, or as an alternative to crest factor reduction, power amplifier distortion may be addressed by amplifier linearization techniques. Known approaches are to “linearize” the power amplifier by placing a preprocessing element between the input signal and the power amplifier. Prior art analog linearization approaches include the use of a non-linear, inverting amplifier in a feed forward architecture which is designed to produce a signal output that has distortion. This signal has distortion that is in direct opposition to the distortion observed in the power amplifier for a given input signal, the inverting distortion is amplified by a second amplifier, and the output is subtractively combined with the output of the power amplifier prior to presenting the signal to the antenna. In this manner, the overall response of the amplifier system as observed at the transmit antenna remains linear. From a complexity standpoint, the analog feed forward inverse distortion function must be very precise over a range of voltages, temperatures and conditions and thus implementing such a system can be costly, and in fact it may not be possible to build a practical amplifier to achieve the desired performance goals. From an efficiency point of view, the added analog amplifier adds additional power consumption similar to the power amplifier itself, which therefore may in fact result in no improvement over the power amplifier alone in a “backoff” configuration.
As an alternative approach to linearizing power amplifiers, digital predistortion circuits (“DPD”) are also known in the prior art and are increasingly used. Typically these are described as implementing algorithms which may be implemented conveniently using existing DSP circuits for example, digital filters in combination with microprocessors and memory devices, or using integrated circuits which implement the algorithm in dedicated hardware as one or more ASICs, gate arrays, or custom integrated circuits. An off the shelf predistortion circuit for use in linearizing an amplifier is sold by Intersil Corporation, designated Intersil part number ISL 5329. The digital predistorters of the prior art typically attempt to create an inverse distortion to that distortion expected or known in the power amplifier, and this inverse distortion is applied on the input signal prior to presenting the signal to the power amplifier. Thus the two amplifier gain stages are operated in a cascade, and the predistorter amplifier provides an “inverse” function to compensate for the distortion expected in the power amplifier. The inverse distortion can be based on a model for the power amplifier which predicts the power amplifier distortion based on the currently observed samples of the incoming signals. Alternatively, a look up table solution can be developed using experimental observations of the power amplifier over a range of expected input signal situations. The predistorter and the amplifier are coupled in cascade to attempt to compensate the overall signal characteristic, and to cause it to be linear over the operating range of the power amplifier.
One disadvantage of the digital predistortion approaches of the prior art is that many current approaches assume the power amplifier is a “memoryless” non-linear amplifier. A memoryless approach begins with the premise that the output of the power amplifier at a given time depends solely on the input signal at that given time, and not on other effects. This may be considered a “0th” order polynomial representation filter. The prior art approach assumes the non-linear element distortion is simple, that is AM-AM (amplitude) and AM-PM (phase) distortion. However, in practice it is known that a power amplifier has significant memory effect performance characteristics. The term “memory” and “memory effects” refers to effects that are non-linear effects in the power amplifier response. These effects may include envelope effects and frequency effects. Envelope effects may be primarily the result of thermal hysteresis and electrical characteristics of power amplifiers. Frequency memory effects are due to variations in the frequency spacing of the input signals and may be characterized by shorter time constants. The temperature of the amplifier at an instant in time may in fact be partly determined by a thermal effect sometimes referred to as “self heating” that occurs during periods of peak power transmission, the temperature also depends on the environment which may include climate effects. Alternatively the amplifier may operate at a cooler than normal temperature for periods of no or little power transmission. The ambient temperature, which may or may not include the effect of operating heating and air conditioning equipment, may change rapidly or slowly and affect the distortion of the power amplifier. The previous signal transmissions and the ambient operating environment therefore create various short and long term memory effects; thus an effective amplifier linearizing method and system must address these long term or memory effects, in addition to the memoryless effects. That is, to effectively linearize the non-linear element (the power amplifier in this case) the design approach and system must incorporate adaptive functions that adapt for memory effects. In fact, it may be practically impossible to create a real time “inverse” amplifier predistortion circuit that can be commercially implemented for certain amplifiers and applications.
FIG. 2 depicts one typical prior art approach to predistortion for a power amplifier. In FIG. 2, power amplifier 25 is depicted as a circuit element, the digital to analog converter, driving transistors, and in the feedback path, the required analog to digital converter, as are shown in FIG. 1 are not depicted in full in FIG. 2, but are included in the box enumerated 25 including the power amplifier denoted “PA”.
Digital predistorter 21 modifies the input signal VIN before it is presented to the power amplifier 25 in a manner intended to invert the distortion that will occur in the power amplifier, and therefore, linearize the output of the overall system. An input signal sequence x(k) is received and is coupled to magnitude squared detecting unit 27 and a random access memory block 33 labeled RAM1. An adaptive device 29 receives the output of the magnitude detecting device 27. Adaptive device 29 may be a look up table (LUT) or alternatively, a more complex polynomial expression unit (POLY) may be used.
The adaptive device 29 provides a predistortion signal to the multiplier 31. The node 31 uses input signal x(k) and the predistortion signal to present a corrected signal VFF to the power amplifier PA, which presents signal VOUT to the antenna or other output device. Note that in this predistortion scheme, it is observed that the predistorter can be made rotationally invariant, thus the adaptive circuit 29 only needs to receive the magnitude squared data of the input signal to affect the signals in the desired manner.
A feedback path provides a feedback form of output signal VOUT (appropriately frequency converted and following analog to digital conversion as shown in FIG. 2,) VFB, to a second random access memory device 35 labeled RAM2. Memory storage devices RAM1, RAM2 are coupled with a processing unit 37 labeled SP which may be, for example, a commercially available digital signal processor (DSP). Other processors such as fixed and floating point processors, reduced instruction set (RISC) machines, multiprocessor devices, and programmable microprocessors such as x86, Pentium, ARM, MIPS and other known processors may be utilized, or a custom processor may be provided. Signal processor 37 uses information relating to the input signal VIN from the memory device RAM1 and correction information relating to the output signal VOUT from the memory device RAM2, and based on difference determined between the two captured stored signals, provides a correction signal to adaptive circuit 29. The correction signal relates to differences between the observed output signal VOUT and input signal VIN, and is based upon the assumption that ideally, the signals VOUT and VIN should be substantially equal except for scaling (the scaling being the desired gain provided by the power amplifier PA) and other intended differences. Undesired differences detected in the comparison between the input signal and the observed output signals are assumed to have been introduced by the predistorting device 21 or the PA 25; these are then sought to be cancelled by the correcting signal.
FIG. 3 is a graphical depiction of the VOUT-VIN curves of the prior art circuits of FIG. 2. The ideal or linear response is represented by the line labeled IDEAL in FIG. 3, the dashed line. The response for a typical power amplifier PA is the solid curved line labeled PA. The possible digital predistortion curve for a predistorting signal is the solid line labeled DPD. The combined response curve that can be expected is the line labeled DPD+PA in FIG. 3.
In FIG. 3, it can be seen that when the output voltage VOUT (vertical or Y axis) exceeds the voltage where the power amplifier transistors are saturated (VSAT) the response will become increasingly non-linear. However, even at voltages below that level, the operation of the power amplifier alone is non-ideal as can be seen from the curves, the response moves away from the linear, IDEAL line at lower operating voltages. However, in the area of the graph labeled “Feasible Operation” it is possible to linearize the amplifier (in a simple case, such as illustrated here) using a digital predistortion function that predistorts the signal, this is seen by comparing the curve labeled DPD+PA with the IDEAL curve, in the “Feasible Operation” region it is the same as the line labeled IDEAL, or in effect, the amplifier is linearized. In the area labeled “Infeasible Operation”, it is not possible to correct the distortion. Thus the use of CFR techniques along with a digital predistortion scheme may be important in order to fully linearize a non-linear element in a particular application, otherwise even with a predistortion function, the system may operate in a non-linear region.
The design approach of a system incorporating a prior art predistortion circuit can be best understood by considering the system as two cascaded gain stages, one for the power amplifier, and one for the predistorter circuit. This arrangement is depicted simply as two blocks in FIG. 4. Here the voltage characteristic of the digital predistorter 21 is depicted having a first voltage transfer characteristic, and the voltage characteristic of power amplifier 25 is depicted having a second voltage transfer characteristic exhibiting the typical non-linear characteristic of a power amplifier.
The design goal of a digital predistortion system is to create a combined linear input-output gain of G for the system, so that the gain F of the predistorter satisfies the function of:F(H(VIN))=G(VIN)=VOUT where H is the gain of the power amplifier, F is the gain of the predistorter, and G is the ideal linear gain of the cascaded system.
With reference now to FIG. 5, there is shown an exemplary basestation system 50 as is known in the prior art for the forward transmission path for existing cellular systems. The power amplifiers in such a system are known to be non-linear elements.
In FIG. 5, network switch 51 is depicted which receives information from a networked system, for example, a typical network fabric in a conventional wired or land line telephony system. A plurality of base band processors 55 are coupled in a parallel fashion to receive and transmit information to and from the network switch 51. Element 57 is a baseband switch distribution block which combines (depicted only in the forward transmit direction, although received signals are also processed) baseband data for the individual communications channels for each baseband processor 55. Radio Card/RFM module 61 includes a formatter, forward transmit processor, data converter for transmission TX RF, and on the receiver side, data converter for received signals RX RF, a digital down converter which may be implemented, as shown here, with Texas Instruments part no. GC5016 programmable up/down converter or an equivalent, and a control processor 59.
Radio Card/RFM 61 includes processing circuitry for processing the baseband signals prior to presentation to the power amplifier and in the prior art may incorporate digital predistortion circuitry. Power amplifier 41, as shown in FIG. 1, is a transmit power amplifier which is the amplification element to be linearized. Duplexer 65 will pass the signals to be transmitted to the antenna 71 and separates the received signals and passes those to tower mounted amplifier (TMA) modulator 67, which is coupled to the receive input of RFM 61. Radio Card/RFM 61 is shown as a single instantiation in FIG. 5; however in a practical system there may be many such cards in a given application as indicated by the replicator dots shown beneath element 61. Power amplifier 41 is simplified for illustration and includes the amplifier and RF transmit and receive circuitry of FIG. 1 for the transmission of analog signals output at antenna 71. Duplexer TX/RX filter 65 combines and separates the transmitted and received signals. TMA block 67 is a tower mounted amplifier that performs the function of amplifying the received signals from the antenna 71. It is recognized that many other elements such as noise filters, bandpass filters, upconverters and downconverters, may be used as is known in the art, however these are not illustrated in FIG. 5 for the purpose of keeping the illustration simple in this explanatory example.
Downconverter GC5016 is a commercially available digital downconverter integrated circuit available from Texas Instruments, Inc. and other similar circuits are available from other vendors as is known to those skilled in the art. This integrated circuit receives digital data from the analog to digital converter in the power amplifier and provides a conversion to a lower frequency, and decimation of the signal required to use the received signal in the baseband processors.
In operation, the system of FIG. 5 receives signals from the Net Switch 51, these are individually processed by the baseband processors 55 as channels, these are then combined for transmit messages, (or separated for receive messages), by the baseband switch/distributor 57, and the signals are provided to one of the radio cards/RFM 61. The signals for forward transmission to a cellular/wireless telephone system, for example, are then provided to the transmit processor in digital form, predistortion and linearizing steps are performed, the data is converted to analog format, up converted and once in the appropriate form, provided to the power amplifier 41 and driven out as a transmission signal on antenna 71. Received signals at the antenna are separated from the transmit signals in block 65, the duplexer, attenuated at TMA block 67 and provided to the analog to digital data converter RX RF, the digital data signals are then down converted by GC5016 integrated circuit or another similar downconverter, and provided through the BP IF/format block to the baseband switch 57, and then separated into channels for processing by the base band processors 55, and the resulting data is placed back in the switched network via switch 51.
Thus, there is a continuing need in the art for a system and method that provides efficient adaptive linearization of a non-linear element, such as a power amplifier for RF transmission. The system and methods should be realizable using commercially available technology. Embodiments and methods of the present invention described below address this need.